Eddy current probes are used to inspect the surfaces of metal and semiconductive objects. In one application, eddy current probes are applied to inspect the surfaces of metallic objects, such as rotor blades of steam and gas turbines. Eddy current probes provide a non-destructive test for inspecting the rotor blades.
Eddy current probes are applied to the surface (or near to the surface) of the metallic or semiconductive object being inspected. To induce a useful eddy current on a conductive surface, the probe applies an electromagnetic field to the surface to induce eddy currents. This electromagnetic field is generated by current in an inducing (driving) coil of the eddy current probe. The eddy currents induced on a surface have relatively low energy and are best detected by probes placed on (or at least near) the surface. A sense coil in the eddy current probe is placed against a surface so that the sense coil current is influenced by the surface eddy currents. The effects on the sense coil currents by the surface eddy currents are measured by processing circuits associated with the probe. To measure the thickness of an insulating coating, the eddy currents on a conductive or semi-conductive surface below the coating is measured. This eddy current measurement is indicative of the coupling between the eddy current probe and the having the coating. This coupling between the probe and part is a function of the distance between the probe coils and the conductive or semi-conductive surface below the insulating coating. Accordingly, the thickness of the coating may be determined from the eddy currents measured by the probe.
Many of the objects to be inspected with an eddy current probe have complex surface contours. For example, a rotor blade has a twisted airfoil column, a flange and a pine tree root. Applying an eddy current probe to the surface contours of a rotor blade or other object is difficult, especially with robot controlled inspection instrument. The tip of the eddy current probe may not conform to the surface of the object being inspected. If good contact is not established between the eddy current probe and the surface, or if there is an air gap between the probe and surface, the probe may be unable to accurately measure the eddy currents induced on the conductive or semi-conductive the surface. Similarly, the tip of the probe may be broken if it is too forceably driven onto the surface. Accordingly, there is a need for an eddy current probe that is sufficiently flexible to conform to odd surface shapes, and that is capable of withstanding impacts with a surface being inspected.